Planetary gears and gear sets are used in a variety of applications, including automotive systems. One example of a planetary gear is a pinion gear rotatably mounted on a shaft via a radial bearing. The shaft is attached on both sides to a carrier, which may also house additional pinion gears. In some applications, planetary gear shafts experience large surface contact pressures and high temperatures. An increasing need for smaller planetary gear sets and components or higher torque inputs in automotive applications has resulted in higher power density and higher contact pressures on the planetary gear shafts.
These planetary gear shafts may require sufficient hardness and strength to withstand surface contact pressures of 5000 MPa and above, while maintaining satisfactory rolling contact fatigue life, and dimensional stability at the elevated temperatures. These qualities are difficult to obtain using cost-effective materials and processes.
In particular, current heat treatment processes used to harden planetary gear shafts do not achieve a necessary hardness when applied to a smaller shaft. Alternative options for achieving localized surface hardness, such as shot peening, plating, coating, etc., do not provide sufficient hardness and/or hardness depth and may not provide quality rolling surfaces for radial bearings. Further, materials that inherently possess the requisite qualities are not economically viable. Thus, there is a need for a process for hardening cost-efficient materials (e.g., steel alloys) capable of producing planetary gear shafts with sufficient hardness qualities to withstand the increased surface contact pressures placed on shafts.
The present disclosure is directed to overcoming these and other problems of the prior art.